1. Field of the Invention
This invention relates to a semiconductor light emission device, and more particularly to a semiconductor light emission device which has a semiconductor light amplifier as a light source and in which light emanating from the semiconductor light amplifier is selected by wavelength and only a selected wavelength is returned to the semiconductor light amplifier, thereby controlling the wavelength of light emitted from the semiconductor light emission device.
2. Description of the Related Art
There have been made various attempts to obtain a high output light beam of a single wavelength by use of a semiconductor. An example of such a semiconductor light emission device is disclosed in "ELECTRONICS LETTERS", vol. 29, No. 14, (1993) pp. 1254 to 1255.
The semiconductor light emission device has a semiconductor light amplifier 1 as a light source as shown in FIG. 11. Light emanating from the rear end face 1a of the semiconductor light amplifier 1 is collimated by a lens 2 and reflected by a reflecting diffraction grating 3 to return to the semiconductor light amplifier 1. With this arrangement, light 4 having a wavelength selected by the diffraction grating 3 is returned to the semiconductor light amplifier 1 and accordingly the wavelength of light 4F emitted through the front end face 1b of the semiconductor light amplifier 1 is locked to a single wavelength, whereby a light beam of high quality and high output (not lower than 1 W) close to a diffraction limit can be obtained.
This applicant has disclosed a semiconductor light emission device in which the wavelength of emission light is selected by a band pass filter in place of the aforesaid diffraction grating, thereby increasing the degree of freedom in selecting the semiconductor light amplifier, reducing the manufacturing cost and facilitating adjustment. See U.S. Pat. No. 5,608,743. As shown in FIG. 12, the semiconductor light emission device comprises a semiconductor light amplifier 1, a mirror 5 which reflects light 4 emanating from the rear end face 1a of the semiconductor light amplifier 1 to return to the rear end face 1a and a narrow band pass filter 6 inserted into the optical path of the light 4.
Though the semiconductor light emission device using such a band pass filter can accomplish the objects, it is disadvantageous in that the threshold current for oscillation is high and the light emission efficiency is low.